1. Field of the Invention
The present invention relates to a crystal oscillator in which a crystal blank and an IC (integrated circuit) chip are accommodated in a vessel and a method of fabricating the same. In more particularly, the present invention relates to a crystal oscillator and a method of fabricating the same in which the IC chip is fixed through a bump to the vessel by means of ultrasonic thermocompression bonding.
2. Description of the Related Art
A crystal oscillator is widely utilized as a device for providing a reference frequency and a reference time in various electronic apparatus including communication equipment. Recently, as is represented by portable equipment such as a portable telephone, the size of the apparatus becomes smaller and smaller. Thus, the crystal oscillator is also requested to be made small. The size requested for the crystal oscillator is, for example, 3 mmxc3x975 mm for the bottom face thereof and 1 mm for the height itself. For this reason, when an IC chip including the crystal oscillator is mounted on a vessel or the like, the manner of mounting is changed from a conventional wire bonding to a face down bonding (FDB) in which one major surface of the IC chip having terminal electrodes formed thereon is brought into the vessel with its face down and bonded to an opposing substrate. As an example of the face down bonding, there has been proposed a bonding method using ultrasonic thermocompression bonding employing a bump.
FIG. 1 is a diagram showing a cross-sectional view of a general arrangement of a crystal oscillator. FIG. 2 is an oblique view showing an outer appearance of the crystal oscillator shown in FIG. 1, and FIG. 3 is a diagram showing a bottom view of the crystal oscillator shown in FIG. 1.
As shown in FIG. 1., the crystal oscillator includes vessel main body 1, IC chip 2 and crystal blank 3 accommodated in vessel main body 1, and cover 4 bonded on vessel main body 1. Vessel main body 1 is formed of bottom wall 5, intermediate frame wall 6 and upper frame wall 7, and thus concave portion 1a and step portion are formed in vessel main body 1. The components constituting vessel main body 1 is made of baked ceramic of a multilayer structure. IC chip 2 has a number of electronic components integrated thereon such as an amplifier and so on constituting an oscillation circuit. On one major surface of IC chip 2, there are provided crystal terminals 8a and 8b utilized when it is connected to crystal blank 3. IC chip 2 is fixed to the bottom surface of concave portion 1a of vessel main body 1 at the one major surface through, for example, bumps 9 by means of ultrasonic thermocompression bonding, in a manner of face down bonding.
As shown in FIG. 4, crystal blank 3 has a substantial rectangular shape and has a pair of excitation electrodes 14a and 14b on both of the major surfaces opposing to each other, respectively. Each of the excitation electrodes 14a and 14b has leading electrode 15a, 15b extended toward the ends opposing to each other in the longitudinal direction of the crystal blank. Crystal blank 3 is fixed to the step portion at both the ends thereof by using conductive adhesive 10. Thus, crystal blank 3 is electrically and mechanically connected to a crystal connection terminal formed at the step portion. Cover 4 is bonded on vessel main body 1 by means of seam welding metal ring 11.
Pair of leading electrodes 15a and 15b of crystal blank 3 extend through a through-hole provided at the step portion of vessel main body 1 and connected to crystal terminals 8a and 8b of IC chip 2. Each of the terminals of power supply, signal output, ground or the like of IC chip 2 is led to the bottom surface or side surface of vessel main body as outer electrode 12. Further, in order to measure electric characteristic of the crystal oscillator and electric characteristic of crystal blank 3 as a unit component, the pair of leading electrodes 15a and 15b of crystal blank 3 are extended to the side surfaces of vessel main body 1 to form measuring terminals 13a and 13b on the side surfaces. In FIG. 1, a portion denoted by a black bold line represents a conductive portion for establishing electric connection such as a through-hole, a conductive path which will be described later on, an electrode, an intetlayer connection or the like.
When the device as a product is shipped, the crystal oscillator contained in the device is subjected to a forcible excitation washing. The forcible excitation washing is carried out as a countermeasure for so-called DLD (Drive Level Dependency) in which crystal oscillator 3 becomes unresponsive in oscillation to a small level of excitation. However, in the arrangement of the above crystal oscillator shown in FIG. 5, measuring terminals 13a and 13b of crystal oscillator 3 are also electrically connected to crystal terminals 8a and 8b of IC chip 2, respectively. For this reason, if a voltage high enough to effect the forcible excitation washing is applied to crystal blank 3 through measuring terminals 13a and 13b, then IC chip 2 can also be applied with the voltage excessively through crystal terminals 8a and 8b, which fact can cause electrical damage on IC chip 2.
General manner of bonding IC chip 2 to a vessel or the like to form a crystal oscillator will hereinafter be described.
As shown in FIG. 6, a plurality of conductive paths 25 as a circuit pattern are formed on a bottom surface, or bottom wall 5 of concave portion 1a of vessel main body 1. A base layer of conductive paths 25 is made of tungsten (W) by printing and burning as a base electrode. Thereafter, a gold (Au) layer is provided on the surface of the base layer by electrolytic plating. In an ordinary manner, conductive path 25 is covered with an insulating material such as alumina or the like (not shown) except for connection terminal portion 26, which serves as an end contact area of conductive path 25, denoted by applying A half-tone notation in FIG. 6. Connection terminal portion 26 is formed into a rectangular shape extending in the longitudinal direction of conductive path 25.
On the other hand, IC chip 2 is supplied as a bare chip or a flip chip. As shown in FIG. 7, IC chip 2 is formed into a rectangular shape having a plurality of terminal electrodes 27 formed along a pair of side edges opposing to each other on one major surface of the chip. Each of terminal electrodes 27 has provided thereon ball-like bump 28 made of a gold grain, for example. When IC chip 2 is mounted in concave portion 1a of vessel main body 1, IC chip 2 is brought into concave portion 1a so that one major surface of the IC chip on which the bumps are provided is opposing relationship with the bottom surface of concave portion 1a and bumps 29 and connection terminal portions 26 are aligned with and bonded to each other. Then, IC chip 2 is pressed and ultrasonic wave is supplied to IC chip 2 by an ultrasonic thermocompression bonding machine so that IC chip 2 is vibrated in the horizontal direction. Thus, bump 28 is crashed and formed into an elliptical shape and electrical connection is established between terminal electrode 27 and connection terminal portion 26 through bump 28. Bonding is achieved by effecting solid phase diffusion in the metal (in this case, gold).
Since crystal blank 3 has a rectangular shape elongated in one direction, the bottom surface of concave portion 1a of vessel main body 1 also becomes elongated rectangular shape. Therefore, if small-sizing of the crystal oscillator is developed, almost no allowance is provided between the side wall of the concave portion and IC chip 2 in the width direction. For this reason, conductive path 5 tends to be provided on both the sides in the longitudinal direction in which certain allowance can be expected. Thus, IC chip 2 is arranged to have terminal electrodes 27 formed on a pair of sides opposing to each other in the longitudinal direction of the chip. Further, the number of terminal electrodes 27 provided along each of the longitudinal sides is arranged to become almost the same (within an extent of 4:6) so that pressing force can be applied uniformly on the IC chip when ultrasonic thermocompression bonding is effected. Further, interval between terminal electrodes 27 is also almost regulated.
Now, IC chip 2 will be further described. As shown in FIG. 8, IC chip 2 is formed of silicon substrate 2a on which amplifiers, resistors, capacitors and so on, which constitute an oscillation circuit, are integrated. On the surface of silicon substrate 2a, there is formed an oxidized film (SiO2) for isolating a P-type region from an N-type region so as to surround the boundary of the frame. Then, an aluminum (Al) film (of which thickness is about 1.2 xcexcm) is provided over the oxidized film and the frame formed of the oxidized film 2b. Then, terminal electrode 27 is formed so that it is electrically connected to the P-type region or the N-type region. The Al film as terminal electrode 27 is formed by vapor deposition or spattering. In this way, terminal electrode 27 is formed into a recess shape having a step at the periphery thereof. As described above, terminal electrode 27 is formed on a pair of sides opposing to each other of a circuit formation surface of IC chip 2. Bump 28 is formed on the bottom surface of the concave portion of the terminal electrode 27. As shown in FIG. 8, bump 28 may be formed in such a manner that two gold grains are piled on one another in the vertical direction.
FIG. 9 is a diagram showing an IC chip having a bump which is brought into a crashed state owing to face down bonding.
However, in the crystal oscillator of the above arrangement, terminal electrode 27 often suffers from crack and a fragment thereof can be cut away therefrom due to a pressing force with vibration when ultrasonic thermocompression is effected. Further, an internal circuit located beneath terminal electrode 27 or vicinity of the same often suffers from damage, with the result that a connecting portion for connecting terminal electrode 27 to connection terminal portion 26 suffers from connection failure (bonding failure). Thus, the crystal oscillator of the conventional arrangement sometimes consumes an expensive IC chip 2 uselessly, which fact leads to bad yield and low productivity.
Further, vessel main body 1 is made of ceramic. Due to the inherent nature of a mold body made of ceramic, it is difficult to configure the vessel at a high accuracy in shape, dimension, flatness or the like. For this reason, it is also difficult to maintain parallelism between the mounting surface of IC chip 2 and the bottom surface of concave portion 1a of vessel main body 1. If the crystal oscillator for surface mount is made to have a small size such as of 5 mmxc3x973 mm, the vessel main body itself is also small and configured at low accuracy. Further, since IC chip 2 cannot be bent with ease, it is far more difficult to mount IC chip 2 on vessel main body 1 having a small area by means of face down bonding than to mount a part module formed of glass-epoxy resin or the like on a substrate having a large area by means of face down bonding. Thus, it is difficult to establish electrical connection at all of the bonding portions.
Therefore, an object of the present invention is to provide a crystal oscillator in which it becomes possible to positively avoid bonding failure between a terminal electrode of an IC chip and a connecting terminal portion formed on a bottom surface of a concave portion of a vessel, and hence productivity is improved.
Another object of the present invention is to provide a crystal oscillator in which an internal circuit of the IC chip can be protected from electrical damage, and hence productivity is improved.
Another object of the present invention is to provide a crystal oscillator in which a crystal blank can be subjected to measurement of various characteristics by applying forcible excitation without electrical damage on the IC chip.
Still another object of the present invention is to propose a method of bonding an IC chip as a fabrication processes for producing crystal oscillators at a low rate of defect.
In order to solve the above-identified problems which a conventional crystal oscillator tends to encounter, we carried out investigation to analyze the cause thereof. The following is a knowledge obtained by us.
Each of the conductive path or connection terminal portion on the bottom surface of the concave portion does not always have a regular width, due to a design circumstance such as the difference of the interval of the terminal electrodes on the IC chip. Further, since the base electrode made of tungsten or the like is formed by printing and baking, the height of the conductive path becomes large when the width thereof is narrow while the height of the conductive path becomes small when the width thereof is wide. For this reason, the height of the connection terminal portion is scattered. For example, if a connection terminal portion having a wide width and a small height is provided between connection terminal portions having a narrow width and a large height, pressing force will not be applied to the connection terminal portion having a wide width and a small height satisfactorily, with the result that contact of the connection terminal portion having a wide width and a small height to a bump becomes unsatisfactory. Thus, solid phase diffusion will not be effected between the bump and the connection terminal portion, which fact tends to lead to connection failure between them.
As described above, the terminal electrodes of the IC chip are formed on both the sides opposing to each other, and in correspondence with the terminal electrodes, the connection terminal portions are formed on the bottom surface of the concave portion. Most of the connection terminal portions are formed so as to extend in a direction perpendicular to the pair of sides along which the terminals are arrayed. However, as shown in FIG. 6, some of the connection terminal portions can be formed so as to extend in a direction parallel with the pair of sides along which the terminals are arrayed.
In this case, consideration is given to the relation between the direction of vibration of an ultrasonic wave upon effecting ultrasonic thermocompression bonding and the direction in which the connection terminal portion extends.
As shown in FIG. 10A, if the direction of vibration of the ultrasonic wave applied to the IC chip (indicated by an arrow P) is made in parallel with the longitudinal direction of connection terminal portion 26 of a substantial rectangular shape provided on the end of conductive path 25, bonding will be satisfactorily achieved regardless of the deviation of the bump. Thus, if the direction of vibration of the ultrasonic wave applied to the IC chip is made perpendicular to the pair of sides on which the terminal electrodes of the IC chip are provided, the direction in which most of the conductive paths 25 (connection terminal portions 26) extend becomes coincident with the direction of vibration of bump caused by the ultrasonic wave. Accordingly, satisfactory bonding can be expected at the connection terminal portions.
Conversely, as shown in FIG. 10B, if the direction in which connection terminal portion 26 extends is perpendicular to the direction P of vibration of the ultrasonic wave and the width of connection terminal portion 26 is narrow, then bump 28 will bulge out of the width of connection terminal portion 26 due to the deviation of bump 28 caused by vibration. Thus, connection failure is caused. It is true that to widen the width of connection terminal portion 26 is a possible countermeasure for preventing connection failure. However, the IC chip is requested to be small-sized and have a desired function, and layout of a circuit pattern is determined depending on the request. Thus, the width of connection terminal portion is limited from the design standpoint.
We propose that a plurality of connection terminal portions are arranged to have an identical width for avoiding connection failure. If the plurality of connection terminal portions are arranged to have an identical width, it is expected that all of the connection terminal portions have a substantially identical height. Thus, connection failure caused by unevenness of the height of the connection terminal portions can be avoided.
We also propose that the connection terminal portions are disposed so that the longitudinal directions of the connection terminal portions are parallel with one another, and that the direction of vibration of the ultrasonic wave applied upon effecting ultrasonic thermocompression bonding is made coincident with the longitudinal direction of the connection terminal portions. With this arrangement, the bump can be prevented from bulging out of the contact area upon effecting ultrasonic thermocompression bonding, and connection failure can be prevented.
Further, we found that, in order to avoid crack or the like caused in the terminal electrode upon effecting ultrasonic thermocompression bonding, it is effective to provide a metal layer as a buffer layer on an aluminum layer as the terminal electrode and form a bump on the terminal electrode through the metal layer. It is preferable for the metal layer to be made of gold and formed by plating. Further, it is also preferable for the gold layer to have a thickness larger than that of the aluminum layer constituting the terminal electrode.
Furthermore, we propose an arrangement in which relaying terminals are provided on the external surface of the vessel main body so as to be electrically connected to a pair of excitation terminals of the crystal blank, and another relaying terminals independent of that relaying terminals are provided on the external surface of the vessel main body so as to be electrically connected to the crystal terminal of the IC chip. With this arrangement, the crystal blank can be subjected to measurement of electrical characteristic or forcible excitation washing if necessary by using the relaying terminals electrically connected to the pair of excitation terminals of the crystal blank without damaging the IC chip due to an excessive voltage.